Published in IET Power Electronics Received on 31st May 2011 Revised on 23rd August 2011 doi: 10.1049/iet-pel.2011.0204 ISSN 1755-4535 Flyback-based high step-up converter with reduced power processing stages G.M.L. Chu 1 D.D.C. Lu 1 V.G. Agelidis 2 1 School of Electrical and Information Engineering, The University of Sydney, NSW, Australia 2 School of Electrical Engineering and Telecommunication, The University of New South Wales, NSW, Australia E-mail: dylan.lu@ee.usyd.edu.au Abstract: This study proposes a non-isolated converter derived from the conventional flyback topology based on the concept of reduced power processing. The proposed converter has higher-voltage gain, improved efficiency and reduced component stresses owing to recycling of leakage energy and clamped power switch voltage when compared with the conventional flyback converter, and yet retains the advantages of circuit simplicity and low cost. A complete analysis including converter operation and its dc and ac characterisations are presented in the paper. Using a 60 W laboratory prototype operating at 100 kHz with 24 V input voltage and 200 V regulated output voltage, experimental evaluation of the efficiency improvement, steady-state and transient-response performance are presented to verify the effectiveness of the proposed converter. 1 Introduction Recent research and development of renewable electrical energy systems [1, 2] using low-voltage energy sources such as fuel cells and solar photovoltaic cells call for high efficiency, high step-up dc–dc converters. For these applications it is desirable to have a voltage gain of around ten or above. A basic boost converter cannot provide such high-voltage gain, even with extremely high duty cycle. In addition, owing to high output voltage, the reverse-recovery current of the output diode will cause excessive switching loss and electromagnetic interference (EMI) issue. Numerous topologies of high step-up converter have been reported in the technical literature. Some examples include: the multi-level boost converter [3], converters with cascaded stages [4] and converters with coupled inductors [5, 6]. Apart from these, high output voltage can also be generated by manipulating the charge transference of capacitor or inductor. Charge pumps, switched-capacitor converters and the Luo converter with voltage-lift technique are typical examples [7–9]. Alternatively, switched-capacitor/switched- inductor structure, voltage doubler/multiplier cells can be inserted in basic dc–dc converters to further boost up the output voltage [10, 11]. The limitation shared by these converters is that all the input power needs to be processed by the power switch, which limits the power efficiency. Some of these converters involve the use of multiple switches and magnetic components with relatively complex circuitries, which leads to higher cost and lowered reliability. Apart from these novel topologies, the conventional flyback converters can in fact easily produce high-voltage gain using high turns-ratio transformer. However, it suffers from several limitations that lower its efficiency and degrade its performance in high step-up applications. The presence of the relatively large leakage inductance generates huge turn-off voltage spikes in the power switch, which results in high-voltage stress on the component and requires the use of snubber circuit to clamp the switch voltage. The leakage inductance also induces ringing in the switch current, leading to further reduction in power efficiency and severe EMI issues. Besides, the discontinuous input current associated with the buck-boost-based behaviour also requires the use of large input capacitor to smooth out the input current ripple. Although the flyback converter seems to be not very appealing considering its limitations in efficiency and component stresses, it offers the advantages of low component count, simple structure and low cost, which are desirable for low-power applications such as portable computers, storage devices and mobile/battery chargers [1]. Therefore it is of practical interest to further improve the performance of the flyback converter while retaining its advantages of simplicity and low cost. In view of that, some research efforts have been spent on further improvements and applications of the flyback topology. For example, active clamping and soft-switching functions have been added to the flyback converter in order to reduce its switching losses and component stresses [12–15]. Some research efforts have targeted the conduction losses by using synchronous rectification [16, 17]. Others have aimed at increasing its efficiency and extending its application by series/parallel connection of more than one flyback converters [18–20] or combining it with other converter topologies [1, 21]. However, most of these improvements are achieved at the expense of using multiple switches and more complex circuits, resulting in increased cost and reduced reliability. This paper proposed a non-isolated dc-dc converter with a high step-up ratio. The proposed converter is modified from IET Power Electron., 2012, Vol. 5, Iss. 3, pp. 349–357 349 doi: 10.1049/iet-pel.2011.0204 & The Institution of Engineering and Technology 2012 www.ietdl.org